Beam forming networks have a wide range of applications. In one application the network is used as a power divider/combiner for distributing radio frequency (RF) and/or microwave signals between one port of the network and a plurality of other ports connected to antenna monopoles.
The concept of an orthogonal beam forming network is well-known in the art as a Fourier matrix or a Butler matrix. FIG. 1 illustrates a known beam forming network 100 which includes four 90 degree hybrids 109, 110, 111, 112 connected in a 4×4 matrix as shown. The matrix has four inputs 105, 106, 107, 108, and four outputs 101, 102, 103, 104.
One application of this network is described in U.S. Pat. No. 5,191,349, “Apparatus and Method for an Amplitude Monopulse Directional Antenna,” the disclosure of which is incorporated herein by reference in its entirety. FIG. 2 is an example of a beam forming network 200 used in a traffic alert and collision avoidance system (TCAS) as depicted in the '349 patent. Network 200 includes four 90 degree hybrids 109, 110, 111, 112 and four inputs 105, 106, 107, 108. Outputs 101, 102, 103, 104 to network 200 are connected to four monopoles A1, A2, A3, A4 of a directional antenna (not shown) through matching networks 205, 206, 207, 208. Using network 200, the antenna provides transmit radiation in a directional pattern and receives signals by comparing the amplitudes of the signals induced in the plurality of antenna monopoles. Such an antenna system is suitable for transmitting and receiving TCAS information using a directional antenna radiation pattern only. However, in some applications an antenna should be capable of alternately forming omnidirectional as well as directional antenna radiation patterns. In the current application, the same antenna also has to be used for Transponder and Universal Access Transceiver (UAT) systems with an omnidirectional antenna pattern during both transmit and receive modes. The conventional 4×4 hybrid matrix, connected to a transmit/receive block by four cables, cannot be used as an omnidirectional antenna without beam steering and phase calibration networks. These additional networks vary the phasing of the transmit signal to provide directional and omnidirectional antenna radiation patterns. The additional networks are required to form the omnidirectional antenna radiation pattern and to minimize phase differences, including antenna cable errors and errors relating to transmission pass differences between the channels. The known systems are therefore very complicated, costly, and are characterized by large sizes and extra weight.
It is therefore an object of the invention to provide a means for alternately forming omnidirectional and directional antenna radiation patterns in an antenna system.
It is another object of the invention to provide a means for alternately forming omnidirectional and directional antenna radiation patterns for a TCAS system, Transponder and UAT systems during transmit and receive modes.
It is still another object of the invention to provide a switched beam forming network without any additional beam steering and phase calibration networks.
A feature of the invention is a 4×4 hybrid matrix and a switching network that can form an omnidirectional antenna radiation pattern and a directional antenna radiation pattern for both transmit and receive modes.
An advantage of the invention is that no complicated beam steering and phase calibration networks are required to form directional and omnidirectional antenna radiation patterns.